Wave-guide mode discriminators



June 19, 1956 KlNG 2,751,561

WAT-GUIDE MODE DISCRIMINATORS Filed Dec. 20, 1950 2 Sheets-Sheet 1 FIG.3

SOURCE m4 VEGU/DE 24) 235 FIG. 4c 7 H640 i+ ii BEND/N6 AXIS //v v/v TopA. P. KING A 7' TORNEY June 19, 1956 A. P. KING 2,751, 61

WAT-GUIDE MODE DISCRIMINATORS Filed Dec. 20, 1950 2 Sheets-Sheet 2 //vl/EN TOR A. R KING A T TURNEY illustrate grooves cut into a relativelythick wall as a structure alternative to corrugated tubing.

Let us consider in greater detail a corrugated tube in which thecorrugations are rectangular in shape and which is illustrated in Figs.4A and 4B. Here the wave guide of nominal radius a is connected seriallywith short lengths of circular wave guide of length w. Thus a series ofrings, 22 and 23, alternating in radius b and a respectively, areconnected together in a seamless fashion by the annular rings 21. Thecircular grooves 24 formed in this manner may be considered as disc orradial transmission lines, whose length l=ba, open at wall or radius aand short circuited by a ring 22 at radius b. As indicated in Fig. 4A,these lines are spaced S distance apart and act as series reactiveloading elements for line of radius a. For the conditions preferablyprevailing in this application, where the series impedance presented bythe radial transmission line is a function of length l(l=ba). As withother lines, the reactance is positive or inductive for radial lines perwavelength (A of the line radius (1. Experiment has shown that issufficient and that a value n=3 approx. is too few. Thus it will beapparent that when n=6 and when w=S,

then

)\B wapprox.

The corrugated wave guides of Figs. 4C and 4D show modifications of theform shown in Fig. 4A which facilitate the manufacture of the tubing. Incommercial manufacture which commonly uses hydraulic pressure methods itis not feasible to produce the sharp corners of Fig. 4A. In the analysisit is assumed however that the dimensions in Figs. 4C and 4D aremodified from those in Figs. 4A and 4B only slightly by the rounding ofthe corners.

Two illustrative applications of corrugated wave guide in a TEmtransmission system will be considered, the first of which is the use ofsuch tubing as an expansion section in a straight wave-guide line toallow for thermal expansion or contraction of the wave-guide line, as inFig. 6.

In a straight section of line no degenerate modes will be generated bythe corrugated section, provided preferably less than for under theseconditions the configuration of the TEM wave is such that it does nottend to excite the radial line elements. Under these circumstances thelength l of the radial line is non-critical. The number of corrugationsemployed dependsupon the amount of expansion to be allowedfor.

In the second example corrugated tubing may be used as either" a fixedor flexible bend to avoid degenerative modes (TMn) which occur in aplain bend. Structure is the same as Fig. 4 except that it is bent orcurved.

It has been found by experiment that bending the tubing to a radius of10 to 12 inches does not appreciably affect'the transmission norintroduce appreciable amounts of the degenerate TMn mode.

The function of the corrugations is to produce a large reactive loadingfactor for the TM1 1 wave component which tends to be produced in aplain bend, and thus to change its phase velocity relative to the TEn1-wave component and thus reduce the coupling between the TEoi and TM11waves.

The elfect of the radial line upon the transmission of 7 I field at thewall is tangential (E.,) and the magnetic field longitudinal (Hz). Thusthe TEo1 wave has no field components to excite the radial lines, andonly a minute penetration of the groove will occur. The presence of thegrooves will make the wave guide appear to the TE01 wave as shown inFig. 5B by the solid line. Since the TEoi wave does not penetrateappreciably in the groove the length l is unimportant, and it does notmatter whether the end at the ring 22 (Fig. 4A) is shorted or not. As aresult the effect of the grooves upon the TEoi wave is practically thesame as a straight pipe without corrugations.

For the TM11 wave however the field at the wall possesses bothlongitudinal electric field Ez and tangential components of magneticfield H and therefore can propagate a wave radially along the disctransmission line. For this wave these grooves act as reactive elementsin series with the line, change the LC ratio of the line and hence thephase velocity of the TM11 wave. In a plain wave-guide bend thedegenerate TM11 mode has the same velocity, but in a corrugated bend thereactive loading elfect is to change this velocity and thus decouple thewave sufiiciently so that there is no TMn wave degeneracy in the bend.

Dimensions and tolerances which have been found to work well in thethree-centimeter wavelength range in the structure of Fig. 4D are200010.010 inches for 2a, 2.705i0.0l0 inches for 211, five convolutionsper linear inch of wave guide, and bending radius of 0.020 inch.

. In this example,

l=ba is in the vicinity of AA, or an odd multiple there:

of, where M is the wavelength in the radial line. For a plurality ofradial lines in adjacent portions of the main line there may be aproximity efiect which will cause the greatest effect to be obtained ata slightly different value of l. The length l which develops the maximumreactive impedance is close to the value wa and for a radial linewherein the length I may be calculated from the Bessel functions asfollows, using the relationship given by S. A. Schelkunoif inElectromagnetic Waves, page 269:

N1(1 um where ,3 is the phase shift in radians per centimeter along theradius.

Thus for the case of 41:1 inch= .54 centimeters, and

free space wavelength x=3.33 centimeters.

3= =1.89 radians per centimeter from which 13a=1.89 2.54:4.79 radiansJi(4.79)=-0.2967 and from which is obtained the value .r, 4..79) 37: JmN1(4.79) Noui Entering the chart of Nu/Jo on page 201 of the same book,with the reciprocal of 1.37, the value of 6b which satisfies this ratiois found to be ,8b=6.46 radians from which and the value of 13a,

l=ba=0.91 centimeter=0.358 inch Thus this calculated value of l isfairly close to the specified depth of the annular groove of theillustrative example of wave guide in Fig. 4D, which is 0332:0010 inch.

Fig. 7 shows an alternative structure which has advantages in the way ofease of manufacture. A single helical fold or groove is used instead ofa plurality of parallel circular ones. While the helical form may have aslight tendency to tilt the wave front of the circular electric wave,this efiect is not objectionable in practice if the pitch of the helixis slight or moderate. This form of wave guide lends itself to goodflexibility and may be manufactured in the manner of the sheathingcommonly used in BX cable from a band or ribbon of metal wound helicallywith an interlocking joint between adjoining edges. This form ofconstruction is also similar to that found in flexible speaking tubes asin dictating machines. The diameter of the tube is, of course, to bedetermined by the wavelength of the electric wave which it is desired totransmit.

The embodiments of the invention that are described herein are to beregarded as illustrative only and not as limiting the invention, whichlatter is applicable to many other materials, physical forms and useswithin the scope of the appended claims.

What is claimed is:

1. In a fully confined wave guide system for circular electric Wavescharacterized by circular field lines coaxial with the guide, a sourceof said circular electric waves, a first substantially straight lengthof fully enclosed hollow conductive wave guide of circular cross sectionconnected to said source, a second substantially straight length offully enclosed hollow conductive wave guide of circular cross section,said first and second straight lengths of wave guide having smooth,substantially cylindrical inner wall surfaces and having theirrespective longitudinal axes displaced so as to be out of alignment witheach other, and a curved length of hollow conductive Wave guide ofcircular cross section joining said straight lengths to form a fullyenclosed hollow conductive wave guide system therewith, said curvedlength of wave guide having circumferential grooves in its inner wallsurface, the depth of said grooves being such that and the spacing ofsaid grooves along the length of said curve being small enough that thepropagational velocity of a degenerate transverse magnetic wave modewithin the curved length of wave guide is rendered materially differentfrom the propagational velocity of the circular electric wave modewithin the said curved length, thereby minimizing wave energy transferfrom the circular electric mode to the degenerate transverse magneticmode.

2. A high frequency electromagnetic wave transmission system comprisinga source of wave energy in the circular electric mode, means forutilizing said circular electric mode energy, and a fully enclosedconductively bounded wave guide transmission line of substantiallycircular cross section connecting said source to said utilizing means,at least one portion of said line being a curved section of wave guidehaving a plurality of transverse annular grooves in its inner wallsurface, said grooves having a radial depth differing from an oddmultiple of quarter wavelengths of said wave energy by less thanoneeighth wavelength, said grooves presenting substantial impedance towave energy in the transverse magnetic mode normal to said curvedsection and substantially no impedance to said circular electric modewave energy.

3. A high frequency electromagnetic wave transmission system comprisinga source of wave energy in the circular electric mode, means forutilizing said circular electric mode energy, and a fully enclosedconductively bounded wave guide transmission line of substantiallycircular cross section connecting said source to said utilizing means,at least one portion of said line being a curved section of wave guidehaving a plurality of transverse annular grooves in its inner wallsurface exceeding in number two per Wavelength of said wave energy, saidgrooves presenting substantial impedance to wave energy in thetransverse magnetic mode normal to said curved section and substantiallyno impedance to said circular electric mode wave energy.

References Cited in the file of this patent UNITED STATES PATENTS2,088,749 King Aug. 3, 1937 2,155,508 Schelkunoif Apr. 25, 19392,395,560 Llewellyn Feb. 26, 1946 2,464,598 Meier et al. Mar. 15, 19492,556,187 Ingalls June 12, 1951 2,557,261 Collard June 19, 19512,563,578 Candee Aug. 7, 1951 2,567,748 White Sept. 11, 1951 2,612,559Jouguet Sept. 30, 1952 2,623,121 Loveridge Dec. 23, 1952 2,632,804Jouguet Mar. 24, 1953 2,659,817 Cutler Nov. 17, 1953 FOREIGN PATENTS869,734 France Feb. 13, 1942 OTHER REFERENCES Huxley, L. G. H.:Waveguides, McMillan Co., published 1947, page: 198-203.

